hiruzen because iruka said he was the strongest to date and the time he said that was after the fourth both lived and dies so he knows about the fourths abilities but acknowledges hiruzens strength. and if he is stronger than the first as well who defeated kakuzu, madara with ems and the nine tails imagine how powerful he was in his prime and he was called a god of shinobi.
What does Iruka know about anything? He is a weak shinobi who can't do anything, why would you trust what he says? I don't think you can trust everything a manga character says because let's face it they all say different stuff. All of the other ninja in the manga always talk about how they thought the fourth was the best and would never be surpassed.
We have seen some of Hiruzen's abilities and we saw an Edo Tensei version of both the first and second. Now I know that they were probably a little stronger than that but Edo tensei does not make you insanely weak. they still had all of their techniques and abilities. So I would say that they were strong but not the best, the Senju just have good genes (Hashirama in particular)
Plus what does Hiruzen have that Minato would not? Hiruzen didn't have any special abilities like the other hokages. He was known for mastering all of the basic ninjutsu of the village, he couldn't use anything that required a KG, or any summons other than his own, or hidde abilities of the clans, or stuff that required an element he didn't have. And he didn't have rasengan or any of it's variants.
Minato is way too fast for Hiruzen, and has all of his ST jutsus and rasengan, and at least one element (probably wind), and his summons are way better than Enma. Plus Minato is the best ever at sealing jutsu, Hiruzen had to resort to Minato's technique just to stand a chance against Oro.
So please tell me what Hiruzen would have done against Minato?
Also if I'm not mistaken when the Tobirama died wasn't it at the hands of a group of renegade shinobi who were fans of the Kin Gin bros? And if I remember correctly there were only twenty of them. So let's see, twenty followers of the Kin Gin bros versus 7(?) konoha shinobi, including the second and future third hokages and Danzo who was considered as a rival for Hiruzen.
Was that not Hiruzen in his prime? Yet he and the current hokage and 5 other high ranking ninja couldn't take out twenty ninjas. Let me see, didn't Minato once take out 50 ninjas at a single time? And fight both A and Bee at the same time and come out on top?